1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems which include a power supply monitor.
2. Description of the Prior Art
Elevator systems are expected to operate satisfactorily under both normal and emergency power conditions, and to shut down when the power supply conditions are not suitable for elevator operation. If the elevator car is driven by a three-phase induction motor, it is important to detect the direction of phase rotation and single phasing of the source. U.S. Pat. No. 3,596,137, which is assigned to the same assignee as the present application, discloses a three-phase power supply monitor responsive to both the direction of phase rotation and single phasing.
When the elevator drive motor is a direct current motor, with the direct current voltage supplied by a motor-generator set, single phasing of the three-phase source applied to the A.C. drive motor of the motor-generator set is commonly detected by a monitor which measures the ripple component of the direct current output voltage provided by a three-phase full-wave bridge. If the magnitude of the ripple component exceeds a predetermined magnitude, a relay is energized which removes the associated elevator car from service. While this monitor is satisfactory on normal A.C. power, its operation on emergency A.C. power is marginal. If the distortion of the A.C. voltage waveform provided by an emergency power generator is severe, the harmonic content of the ripple component is increased and the relay will be falsely energized, preventing the elevator car from operating.
When the elevator drive motor is a direct current motor and the direct current voltage is provided by a solid state controlled bridge rectifier, commonly called a converter, the hereinbefore mentioned ripple component monitor becomes even more unreliable upon operation from an emergency power source. The combined effect of the emergency power voltage distortion and commutation notches from the solid state rectifier greatly increases the probability of false operation.
The solid state converter power supply for an elevator system also has a disadvantage when short power voltage disturbances occur, which disadvantage is not present when the direct current voltage is provided by a motor-generator set. The inertia of the rotating mass of a motor-generator set enables it to simply ride over short interruptions or other unimportant disturbances in the three-phase source. A solid state converter, on the other hand, cannot discriminate between those interruptions and disturbances which should be ignored, and those which require prompt action in order to prevent misfiring. For example, if the three-phase source for a dual bridge converter is interrupted while the elevator car is moving, and then the source, or an alternate source, reappears, the synchronization of the gate drive logic will have been lost, and the velocity error may call for opposite polarity current to that being supplied just prior to the outage. Thus, controlled rectifiers of one of the converter bridge circuits may be fired while those of the other converter bridge are conducting, and unless the two banks are split and four costly reactors provided, the fuses will be blown, resulting in stoppage of the elevator car without regard to floor level. The elevator car will remain in this position until the fuses are replaced.
Thus, in order to provide elevator operation under various power supply conditions, it would be desirable to have a three-phase monitor for the alternating potential portion of the power supply of the elevator system which has all of the following characteristics:
(1) It will detect single phasing of a three-phase full wave bridge rectified direct current power supply,
(2) It will operate reliably under normal and emergency power conditions,
(3) It will detect loss of the three-phase source, or any phase thereof,
(4) It will operate with a high degree of immunity to voltage distortion and commutation notches caused by an emergency power generator and controlled rectifier apparatus, and
(5) It will initiate an orderly shutdown or startup sequence for elevator drive apparatus, depending upon the condition of the power supply.